The present invention relates generally to the field of electrical devices, such as circuit breakers and more particularly to a power quality indicator that monitors harmonics in an electrical circuit.
In general the function of a circuit breaker is to electrically engage and disengage a selected circuit from an electrical power supply. This function occurs by engaging and disengaging a pair of operating contacts for each phase of the circuit breaker. The circuit breaker provides protection against persistent overcurrent conditions and against the very high currents produced by short circuits. Typically, one of each pair of the operating contacts are supported by a pivoting contact arm while the other operating contact is substantially stationary. The contact arm is pivoted by an operating mechanism such that the movable contact supported by the contact arm can be engaged and disengaged from the stationary contact.
There are two modes by which the operating mechanism for the circuit breaker can disengage the operating contacts: the circuit breaker operating handle can be used to activate the operating mechanism; or a tripping mechanism, responsive to unacceptable levels of current carried by the circuit breaker, can be used to activate the operating mechanism. For many circuit breakers, the operating handle is coupled to the operating mechanism such that when the tripping mechanism activates the operating mechanism to separate the contacts, the operating handle moves to a fault or tripped position.
To engage the operating contacts of the circuit breaker, the circuit breaker operating handle is used to activate the operating mechanism such that the movable contact(s) engage the stationary contact(s). A motor coupled to the circuit breaker operating handle can also be used to engage or disengage the operating contacts. The motor can be remotely operated.
A typical industrial circuit breaker will have a continuous current rating ranging from as low as 15 amps to over 1200 amps. The tripping mechanism for the breaker usually consists of a thermal overload release and a magnetic short circuit release. The thermal overload release operates by means of a bimetallic element, in which current flowing through the conducting path of a circuit breaker generates heat in the bi-metal element, which causes the bi-metal to deflect and trip the breaker. The heat generated in the bi-metal is a function of the amount of current flowing through the bi-metal as well as for the period of time that that current is flowing. For a given range of current ratings, the bi-metal cross-section and related elements are specifically selected for such current range resulting in a number of different circuit breakers for each current range.
An industrial circuit breaker may also be provided with an electronic trip unit. The electronic trip unit senses overcurrent with amplification circuits which provide corresponding analog inputs to a microprocessor controllers like the bi-metallic element trip unit, the electronic unit will cause a time-delay trip as a function of overcurrent magnitude.
In the event of current levels above the normal operating level of the thermal overload release, it is desirable to trip the breaker without any intentional delay, as in the case of a short circuit in the protected circuit, therefore, an electromagnetic trip element is generally used. In a short circuit condition, the higher amount of current flowing through the circuit breaker activates a magnetic release which trips the breaker in a much faster time than occurs with the bi-metal heating. It is desirable to tune the magnetic trip elements so that the magnetic trip unit trips at lower short circuit currents at a lower continuous current rating and trips at a higher short circuit current at a higher continuous current rating. This matches the current tripping performance of the breaker with the typical equipment present downstream of the breaker on the load side of the circuit breaker.
In certain situations, it may be advantageous to disconnect an electrical system by opening a circuit breaker in the circuit. Such circumstances can include applications for maintenance and control. It may also be used in applications to prevent use of electrical equipment under a specified or selected voltage.
In the case of the circuit breaker having an electronic trip unit (ETU), the ETU measures the electrical current in each of the phases of the electrical circuit and implements an algorithm based on the dissipation of energy within the electrical system. The algorithm accuracy within the ETU is very dependent upon samples taken during current wave forms cycle to determine I2T. In some cases, the number of samples taken do not take into account the heating of the lines that is generated by the higher than the 7th harmonic. It is known that the microprocessors in current ETU's typically sample current wave forms at 32 samples per cycle, which would include as high as the 15th harmonic of the wave forms of the electrical system being monitored.
Other means for controlling power flow in power circuits, for example, non-mechanical apparatus, such as SCRs, may benefit from monitoring harmonic content in such power circuit.
Thus there is a need for a power quality indicator monitoring the harmonics in an electrical circuit protected with circuit breakers having ETU capability to alert an operator of excess harmonics present in the system. There is further need for a device and method for determining harmonic content in power circuit and providing a signal proportional to the degree of harmonic content in the power circuit and providing the signal to an electrical device or indicator.